poses, at a moderate rate of combustion, say from seven to eight pounds of coal per hour on each foot of grate surface, and with the connection with the up-take below the tube sheet, gives results hard to beat. Of course, the exposed surface should be thoroughly clothed to prevent radiation. Objection is sometimes made that this boiler requires closer attention as to water than some others, but this is hardly worth considering in the light of an objection. Neither are they particularly liable (as is often asserted) to failure in their fire boxes when properly constructed. With extremely bad water or very poor coal, their use is not commendable. Their first cost is not much more than the return tubular (including brick work), while the cost of maintenance is about the same.

The greater loss of heat in radiation from masonry will generally render the return tubular scarcely as economical as the locomotive boiler, but the greater simplicity of the former will, perhaps, generally overbalance this loss. In extreme cases of bad water, or in out-of-theway places, in the event of repairs being necessary, the single, or two-flue boiler, or even the plain shell with no flues, will be used; not with the expectation, however, of obtaining high economic results. Whenever, as in the majority of instances, the economy consists in evaporating the greatest amount of water with the least

amount of coal, the result will be reached by providing large (long) heating surface, slow combustion, and ample room for the escape of steam and the circulation of water. Undoubtedly, this means increased first cost, and if it cannot be shown to pay, is not worth consideration. Fortunately, the means employed to insure economy in the use of coal are the very means that should be employed to bring about the same result in regard to repairs, safety, and renewing.

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CHAPTER V.

ECONOMY IN THE USE OF STEAM.

ECONOMY in the use of steam requires that it shall be admitted to the cylinder at or near the beginning of the stroke of the piston, at a pressure not much below that in the boiler; that this pressure shall be maintained practically constant for a portion of the stroke, and that then the supply shall be cut off quickly, the remaining portion of the stroke being completed under the expansion of steam in the cylinder. Under these conditions the pressure of steam in the cylinder will be so far reduced at the conclusion of the stroke that little or no more useful work can be performed by it.

Then this steam must be exhausted so freely that back pressure in the cylinder will be reduced to a minimum.

Whatever tends to prevent in any degree the attainment of these ends, to that extent reduces the gain otherwise attainable by working steam expansively, and brings about an important loss due to the effort required to expel the steam from the cylinder after it has done its work.

To secure these ends, which underlie economy

in the use of steam in the steam-engine, it is imperative that the ports shall be of suitable size; that the steam port shall be quickly opened for the admission of steam; that ample opening shall be maintained up to the point of cut off, and that then (the piston being in rapid motion) the port shall be so nearly instantaneously closed as practically to avoid what is technically termed wire-drawing at cut off.

It is also equally important that the exhaust valve shall open a free passage for the escape of the steam just previous to the termination of the stroke; that this amplitude of opening shall be maintained until near the termination of the return stroke, and that then the exhaust closure shall be reasonably rapid, securing the advantages due to compressing the imprisoned steam.

The important ends of rapid valve opening and closure, and the maintenance of ample effective port area, are accomplished by the wrist plate motion peculiar to the Corliss valve-gear, and by the form of valve employed in a way seldom successfully accomplished by other means; the location and construction of the valves is such that there is no difficulty in securing the long and large ports, which are essential.

At the same time, clearance-a necessary evil, always representing loss-is reduced to a minimum. Numerous attempts have been made, some of them very ingeneous, to devise a valve

gear that should perform its functions as well as this, but they have generally failed. For reasons, amongst others that have been previously named, the Corliss type of engine has for a quarter of a century been used for illustrating economy in the use of steain.

BALANCED VALVES.

The attempt to construct a balanced steamvalve that should not waste steam by leakage, or be otherwise unsatisfactory, has nearly always failed.

But in many instances the use of such a valve, with its parts and pieces subject to derangement, is a necessity, and its objectionable features must be taken as a matter of course. This is the case when the governor, in its efforts to control the speed, must move a heavy valve: It is also generally true in the instance of separate expansion valves working on the back of a main slide, because otherwise the pressure will be so excessive as to rapidly destroy the valve and seat, to say nothing of severely taxing the parts through which motion is transmitted to the valve. No such necessity exists in the use of the Corliss type of valve. The ease with which these valves work may be demonstrated by observing how readily they are moved by hand while under full steam pressure.